Processes for preparing pure diaryl carbonates from monophenols and phosgene are known. The preparation of diaryl carbonates (e.g. diphenyl carbonate, “DPC”) is usually carried out by means of a continuous process, by preparation of phosgene and subsequent reaction of monophenols and phosgene in an inert solvent in the presence of alkali and a nitrogen catalyst in the interface.

The preparation of diaryl carbonates by, for example, the phase interface process is described in principle in the literature, see, for example, in Chemistry and Physics of Polycarbonates, Polymer Reviews, H. Schnell, Vol. 9, John Wiley and Sons, Inc. (1964), pp. 50/51.
In the phase interface process, the starter materials dissolved in solvents and water are reacted with one another. The disadvantage of these processes is the separation of the diaryl carbonate from the solvent by distillation and the renewed working up thereof, and also the sodium chloride-containing aqueous phase as waste product, for which there are only limited possible uses and which may require very complicated work-up steps.
For this reason, processes for the direct phosgenation of monophenols, in which the starting materials phosgene and monophenol are reacted not in a phase interface process in the presence of alkali metal hydroxide solution but in the melt in the presence of catalysts, preferably without use of additional solvents, to form diaryl carbonates and hydrogen chloride instead of sodium chloride, have been developed.

Thus, for example, U.S. Pat. No. 2,362,865 (A) describes a process for preparing diaryl carbonates by direct phosgenation of monophenols at temperatures of from 170° C. to 250° C. using Al phenoxides or Ti phenoxides, but no recirculation of the catalyst, nor is a separation method described.
Both EP 2 371 806 A and EP 2 371 807 A likewise describe processes for preparing diaryl carbonates by direct phosgenation of monophenols at temperatures of from 20° C. to 240° C. using metal halides or metal phenoxides. Recirculation of the catalyst into the process has likewise not been described.
EP 1 234 845 A likewise describes the reaction of monophenols in the melt at temperatures of from 120° C. to 190° C. with a particularly pure phosgene. Nitrogen-containing compounds, e.g. pyridine in amounts of from 0.1 to 10 mol %, based on monophenol used, are employed as catalysts. This publication, too, gives no indication of recirculation of catalyst into the process. Pyridine forms a relatively nonvolatile salt (boiling point: 222-224° C.) with hydrogen chloride and this cannot be distilled off readily. According to the teaching of EP 1 234 845 A, the reaction mixture is therefore firstly neutralized with sodium hydroxide, so that a mixture of water, free pyridine and excess phenol can be distilled off.
WO 2011007001 describes the desublimation of pyridine hydrochloride from a reaction mixture. For this purpose, dichlorosilane-pyridine adduct is heated to 200° C. in a full oil pump vacuum, with pyridine hydrochloride being volatilized and precipitating as a solid.
In addition, there are a number of further patents such as WO 2008/114750 A1, JP 2008-231073 A, JP 2009-114195 A, JP 09-278714 A, JP 09-100256 A, JP 10-245366 A, JP 11-012230 A in which the reaction of monophenols in the melt with phosgene to form diaryl carbonates in the presence of homogeneously soluble nitrogen-containing catalyst is described.
JP 10-077250 A, JP 09-24278 A and EP 1 234 845 A refer to possible recirculation of catalyst, but without making mention of a specific separation of catalyst from the product and a work-up method for the catalyst with a view to recirculation thereof. In addition, reference is made to introduction of aqueous solutions, in particular water and/or sodium hydroxide solution, during the course of neutralization and washing of the reaction mixture.
U.S. Pat. No. 5,239,106 teaches the separation of diphenyl carbonate from catalyst-containing reaction solutions by crystallization of the 1:1 adduct with phenol. However, isolation and recirculation of catalyst is not described here.
None of these publications provide satisfactory indications of methods for recirculation of the catalyst, e.g. pyridine, into the process. In particular, the catalyst is separated off via the aqueous phase after a neutralization step using an aqueous, alkaline solution.
In particular, the prior art does not give any concrete examples of a process for separating the catalyst from the product-containing stream, in which neutralization of the hydrochloride by means of water-containing additions, which is encumbered by the above-described disadvantages, is not carried out as in the above-cited documents.
None of these publications describes a completely water- and wastewater-free process for preparing diaryl carbonates.
The processes known from the prior art are therefore not able to satisfy the demanding economic and ecological requirements in respect of catalyst recirculation and additionally ensure high purities of the products, which in turn are starting materials for further chemical processes.
However, economic aspects have to be taken into account for an industrial process. Recirculation of the catalyst is among the important aspects which are assessed at this point, since a high degree or complete discharge of the catalyst means an economic disadvantage and leads to undesirable pollution of the environment. The wastewater formed has to be purified with a very high outlay, which represents a great challenge for the water treatment works. In the processes of the prior art, either a high technical outlay is necessary in order to make recirculation of the catalyst possible or partial or complete discharge of the catalyst is provided. In both cases, an additional wastewater stream is provided.
In a direct phosgenation process, the provision of efficient recirculation of the catalyst is of the highest importance. Furthermore, the use of aqueous solutions should be avoided wherever possible, not only during the reaction but also in the work-up. This is because wastewater containing organic substances firstly has to be purified in a complicated fashion and then be disposed of.